Combustion effluents and waste products from various installations are a major source of air pollution when discharged into the atmosphere. One particularly troublesome pollutant found in many combustion effluent streams is NO.sub.2, a major irritant in smog. Furthermore, it is believed that NO.sub.2 undergoes a series of reactions known as photo-chemical smog formation, in the presence of sunlight and hydrocarbons. The major source of NO.sub.2 is NO, which to a large degree, is generated at such stationary installations as gas and oil-fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass furnaces, cement kilns, oil field steam generators, and gas turbines.
Various methods have been developed for reducing the concentration of nitrogen oxides in combustion effluents. One such method which was developed is a non-catalytic thermal deNO.sub.x method disclosed in U.S. Pat. No. 3,900,554, to Lyon, which patent is incorporated herein by reference. The process disclosed in that patent teaches the reduction of NO to N.sub.2 by injecting ammonia into the combustion effluent stream at a temperature from about 975.degree. K. to about 1375.degree. K. The examples provided in U.S. Pat. No. 3,900,554 show the reduction of NO by NH.sub.3 at reaction times in the range of 0.075 to 0.20 sec. and provides no teaching as to how optimum NO reductions may be obtained in situations in which much shorter reaction times are available.
Since the issuance of U.S. Pat. No. 3,900,554, there has been a proliferation of patents and publications relating to the injection of ammonia into combustion effluent streams for reducing the concentration of NO. It is the general consensus of the literature that ammonia injection at temperatures greater than about 1375.degree. K. would result in the generation of NO from ammonia. Consequently, conventional selective noncatalytic NO.sub.x reduction processes are practiced by injecting ammonia at temperatures lower than about 1375.degree. K. Because of this temperature limitation, it is difficult, and sometimes not possible, to apply conventional non-catalytic NO.sub.x reduction processes.
This temperature limitation and the failure of U.S. Pat. No. 3,900,554 to teach how optimum reduction may be obtained are important with respect to gas turbines. In gas turbines, the common practice is to burn the fuel in a combustor with quantities of primary air which are slightly in excess of stoichiometric. This produces combustion effluents whose temperature generally exceeds the maximum temperature the turbine blades can tolerate. Consequently, it is common practice to dilute the primary combustion effluents with secondary air in order to bring their temperature down to the level the blade can tolerate. Since gas turbine efficiencies improve with increasing gas temperatures, there is an economic driving force to increase the temperature of operation. This is offset by the fact that blades capable of tolerating higher temperatures have to be made of exotic expensive materials or conventional blades must be provided with expensive cooling systems. Consequently, turbine designs are usually compromises between these conflicting requirements.
The rate of flow through gas turbines is typically such that the time between mixing primary combustion effluents and the secondary air and the passage of this mixture throught he turbine blades is typically only a few milliseconds, much less than shown in the examples of U.S. Pat. No. 3,900,554.
During their passage through the turbine blades, the combustion effluents are greatly cooled. U.S. Pat. No. 3,900,554 contains the further restriction that the combustion effluents must be at a temperature greater than about 1144.degree. K. when NH.sub.3 alone is used, or greater than about 977.degree. K. when NH.sub.3 is used in admixture with a second combustible such as hydrogen. Since the combustion effluents are typically well below these temperatures, one cannot apply the process of U.S. Pat. No. 3,900,554 to gas turbine exhaust in most instances.
In an attempt to overcome the limitations of U.S. Pat. No. 3,900,554, Hishinuma, Aimoto, Azuhata, Nakajma, Uchiyama, Oshima, and Kato (ASME publication 79-GT-69) have disclosed a process in which NH.sub.3 is used in admixture with H.sub.2 O.sub.2. This approach, however, has a severe disadvantage due to the high cost of the H.sub.2 O.sub.2 consumed.
It is also to be noted that the application of U.S. Pat. No. 3,900,554 to gas turbines has been discussed by C. P. Fenimore (Comb. and Flames, 37, 245-250 (1980)). In this article Fenimore explaines that when NH.sub.3 reacts in the presence of NO and O.sub.2, reactions occur whereby NH.sub.3 reduces NO to N.sub.2 and H.sub.2 O and whereby NH.sub.3 oxidizes to form NO, the balance between these reactions is dictated by the temperature. While increasing the temperature increases the rate at which NH.sub.3 reacts, it also shifts this balance, i.e. shifts the selectivity of the reaction, unfavorably. For reduction of NO within the gas turbine, very rapid reaction is required because of the short time the combustion effluents spend between the combustor and the turbine blades, but favorable selectivity is also required. Fenimore concludes that his work suggests that it might be difficult to meet both requirements simultaneously in gas turbines.
Therefore, there is still a need in the art for methods of practicing non-catalytic NO.sub.x reduction processes which will overcome, or substantially decrease, the limitations of non-catalytic deNO.sub.x with respect to gas turbines.
This need takes several forms. The U.S. Federal Standard for gas turbines is defined in terms of the lb of NO.sub.x emitted per unit of fuel burned and is equivalent to allowing the emission of 75 ppm NO.sub.x in combustion effluents diluted with enough secondary air so that the O.sub.2 level is 15%.
While this standard can be met by careful design and operation of the gas turbine and by burning only clean fuels, i.e. fuels with little or no chemically bound nitrogen, situations may occur in which gas turbine operators find themselves about 75 ppm NO.sub.x and need a means of trimming their NO.sub.x emissions.
It is also to be noted that the world's supply of clean fuels, such as natural gas, is finite and as this supply dwindles there is a need to utilize available fuels such as liquids which may be derived from coal and oil shale. Such liquids are high in chemically bound nitrogen and consequently form relatively large amounts of NO when they are burned. A supplier of new gas turbines may wish to modify the design of his turbines to allow them to meet NO.sub.x emission standards while burning fuels of significant nitrogen content, even though such modification may be expensive.
It is also to be noted that under the so-called "bubble" concept, the operator of a number of combustion devices may have the option of controlling the emissions of one or more units to levels well below the levels permitted by the regulations. This would permit him to operator other units at levels above regulation provided the total emissions for all units remains below regulation. Thus, the operator of a gas turbines may have the option of increasing the No.sub.x emissions of some turbines by switching to a nitrogen containing fuel provided he has other units whose emissions can be decreased. Naturally, in such a situation, there is a need to minimize the increase in NO.sub.x emissions the gas turbines using the nitrogen containing fuel.